Betz Limit

Significance

No turbine can take more than 59.3 % of the energy in the wind passing through it, because air that had given up all its energy would stop and block the rotor. What makes the number worth knowing is how little goes into it: Betz treats the rotor as an actuator disc and does nothing but bookkeeping — mass in equals mass out, thrust is the momentum the stream loses, power is the kinetic energy it loses — with no aerofoil, no drag coefficient and no blade count anywhere in the derivation. That is why 16/27 binds every design ever proposed. Good modern turbines reach 45–50 %, close enough to the ceiling that the improvements now are in size and reliability, not the physics.